This invention relates to a method of making metal bodies having the desirable characteristics of a conventional sandwich structure. More particularly, it relates to a method of making metal bodies having integral lightweight, porous metal cores and solid metal facings, that possess the advantages of both conventional solid wrought metal and sandwich metal structures.
Many structural uses of metal benefit from light weight. For example, industries especially benefitting from lightweight metal structures include the space industry, aircraft industry, ship building industry, the medical implant aspect of the health care industry, as well as commercial industries using porous materials for, e.g., filtration, heat exchangers, chemical cells, heat pipes, and pumps. Because of this need for lightweight metal materials, it has long been sought to develop new lightweight variants of known structural metallic elements through the use of composite materials and innovative structural designs.
For example, composites have involved the combination of metallic materials with ceramic particulate, whiskers, and continuous non-metallic fibers, creating certain weight reduction improvements in the resulting metal composite materials. In general, these composite materials suffer from many drawbacks: some are prohibitively costly to produce for many applications; others are not easily formed; and others exhibit incompatibilities between their constituents that limit their structural utility.
Another approach to producing lightweight structural components has been through innovative structural designs. One well-known design is the so-called sandwich structure, involving a pair of face sheets (e.g., solid metal sheeting) with an intermediate and separate core of lightweight material such as sheet-metal "honeycombs," foamed plastics or metals, or the like. Conventionally, such products are formed by producing individual sandwich face sheets in the desired shape, separately producing a lightweight core material, machining the core material in the desired shape, assembling the individual face sheets and core materials, and finally joining the core to the face-sheets by brazing, adhesive bonding, or other conventional joining methods.
This conventional method of forming sandwich structures results in individual components that possess high strength and low weight, due to their solid faces and lightweight cores. Historically, weight reductions of up to about fifty percent are possible with sandwich structures, when compared to solid components used in the same structural applications. Despite such low weight, sandwich structures still have relatively high strength because the bulk of their mass is in their solid outer surfaces, where structural loads are most severe. Therefore, components built from sandwich structures can, like solid metal components, carry high structural loads, withstanding bending, buckling and compression.
While conventional sandwich structures provide the foregoing advantages, they also possess certain disadvantages. Because the conventional methods of forming sandwich structures involve separate construction and bonding of the face sheets and the core material, their manufacture frequently involves higher construction costs when compared to the processing of conventional solid metal structures. In addition, since they are not integrally formed, conventional sandwich structures have lower degrees of structural stability as compared with solid wrought metal components, and may be prone to oxidative attack and delamination of the separate face sheet and core components thereof.
Conventional, solid metal structural components, despite their advantages, such as affordable machining and forming, and increased stability, have their own unique disadvantages. Most notably, solid components have poor weight efficiency in structural applications since the solid mass at the core of the component is not fully utilized to carry load.
The patent literature describes numerous techniques for producing metal structures combining solid and sandwich-type structural elements. For example, methods of forming porous metal foams are described in Elliott U.S. Pat. No. 2,751,289; Pashak U.S. Pat. No. 2,935,396; Allen et al U.S. Pat. No. 3,087,807; and Patten U.S. Pat. No. 4,099,961. Techniques for forming composite homogeneous and sandwich-type metallic products are disclosed in Mote et al U.S. Pat. No. 3,135,044; Darling U.S. Pat. No. 3,171,195; Byrne et al U.S. Pat. No. 3,184,840; Trenkler et al U.S. Pat. Nos. 4,434,930 and 4,538,756; Bampton U.S. Pat. No. 4,820,355; and Yasui et al U.S. Pat. No. 5,289,965. These and other disclosures are subject to various of the disadvantages inherent in both conventional solid and sandwich-type metal structural components, some of which are identified above.
U.S. Pat. No. 4,181,549 to Shapovalov describes a technique for preparing an integral metal product that can have a solid metal facing and internally porous core. The technique involves the casting of a molten metal within a vacuum furnace with the simultaneous injection of hydrogen gas into the molten core to form a solid/gas composite. A eutectic reaction produces a single solid phase plus a gaseous phase rather than two solid phases, the hydrogen being rejected from the melt as the metal solidifies to produce pores in the solidified metal. The resulting porous billets can be subsequently converted to desired shapes by deformation processing.
The foregoing method however is only applicable to the formation of porous products employing metals which do not form hydrides under normal processing conditions. For example, due to deleterious hydride formation with titanium, the method is not useful in the production of sandwich-type titanium structures such as are desired for many applications, e.g., for aircraft airframes.
Moreover, as the porous billet of the Shapovalov technique is formed prior to any desired deformation processing, any such further processing may result in partial collapse or other destruction of the desired internal porosity.
Also, the Shapovalov technique requires melting of the metals used, which can have undesirable effects on the structural integrity of certain metal alloys, further making the Shapovalov technique unsuitable for many applications. For example, in the aerospace industry, aluminum is often desirably alloyed with copper, magnesium or zinc, which alloyed aluminum products have significantly higher strength than pure aluminum. Alloy elements, such as copper, magnesium and zinc, however tend to segregate themselves, if the alloyed metal is permitted to remain in the molten state for more than even several seconds, thus destroying the high strength properties associated with the alloyed metal. Thus, as the Shapovalov technique requires processing of the metal or metal alloy in the liquid state for perhaps several minutes, there is a loss of the enhanced strength of any alloyed metal subjected to the method of Shapovalov.
It is among the objects of the present invention to provide a new method for forming integral metal bodies having solid facing and porous cores, which possess the respective advantages of both conventional sandwich structures and solid structural materials and which are not subject to many of the disadvantages thereof. These and other objects and advantages of this method will be apparent from the following description of preferred embodiments thereof.